Ionic dependence of slow waves and spikes in intestinal muscle

A myogenic motor pattern in mice lacking myenteric interstitial cells of Cajal explained by a second coupled oscillator network

The interstitial cells of Cajal associated with the myenteric plexus (ICC-MP) are a network of coupled oscillators in the small intestine that generate rhythmic electrical phase waves leading to corresponding waves of contraction, yet rhythmic action potentials and intercellular calcium waves have been recorded from c-kit-mutant mice that lack the ICC-MP, suggesting that there may be a second pacemaker network. The gap junction blocker carbenoxolone induced a "pinstripe" motor pattern consisting of rhythmic "stripes" of contraction that appeared simultaneously across the intestine with a period of ~4 s. The infinite velocity of these stripes suggested they were generated by a coupled oscillator network, which we call X. In c-kit mutants rhythmic contraction waves with the period of X traveled the length of the intestine, before the induction of the pinstripe pattern by carbenoxolone. Thus X is not the ICC-MP and appears to operate under physiological conditions, a fact that could explain the viability of these mice. Individual stripes consisted of a complex pattern of bands of contraction and distension, and between stripes there could be slide waves and v waves of contraction. We hypothesized that these phenomena result from an interaction between X and the circular muscle that acts as a damped oscillator. A mathematical model of two chains of coupled Fitzhugh-Nagumo systems, representing X and circular muscle, supported this hypothesis. The presence of a second coupled oscillator network in the small intestine underlines the complexity of motor pattern generation in the gut.NEW & NOTEWORTHY Physiological experiments and a mathematical model indicate a coupled oscillator network in the small intestine in addition to the c-kit-expressing myenteric interstitial cells of Cajal. This network interacts with the circular muscle, which itself acts as a system of damped oscillators, to generate physiological contraction waves in c-kit (W) mutant mice.

Expression and function of the Scn5a-encoded voltage-gated sodium channel NaV 1.5 in the rat jejunum

BACKGROUND

The SCN5A-encoded voltage-gated sodium channel NaV 1.5 is expressed in human jejunum and colon. Mutations in NaV 1.5 are associated with gastrointestinal motility disorders. The rat gastrointestinal tract expresses voltage-gated sodium channels, but their molecular identity and role in rat gastrointestinal electrophysiology are unknown.

METHODS

The presence and distribution of Scn5a-encoded NaV 1.5 was examined by PCR, Western blotting and immunohistochemistry in rat jejunum. Freshly dissociated smooth muscle cells were examined by whole cell electrophysiology. Zinc finger nuclease was used to target Scn5a in rats. Lentiviral-mediated transduction with shRNA was used to target Scn5a in rat jejunum smooth muscle organotypic cultures. Organotypic cultures were examined by sharp electrode electrophysiology and RT-PCR.

KEY RESULTS

We found NaV 1.5 in rat jejunum and colon smooth muscle by Western blot. Immunohistochemistry using two other antibodies of different portions of NaV 1.5 revealed the presence of the ion channel in rat jejunum. Whole cell voltage-clamp in dissociated smooth muscle cells from rat jejunum showed fast activating and inactivating voltage-dependent inward current that was eliminated by Na(+) replacement by NMDG(+) . Constitutive rat Scn5a knockout resulted in death in utero. NaV 1.5 shRNA delivered by lentivirus into rat jejunum smooth muscle organotypic culture resulted in 57% loss of Scn5a mRNA and several significant changes in slow waves, namely 40% decrease in peak amplitude, 30% decrease in half-width, and 7 mV hyperpolarization of the membrane potential at peak amplitude.

CONCLUSIONS & INFERENCES

Scn5a-encoded NaV 1.5 is expressed in rat gastrointestinal smooth muscle and it contributes to smooth muscle electrophysiology.

Cellular mechanisms of myogenic activity in gastric smooth muscle

In many regions of the intestine, a thin layer of interstitial cells of Cajal (ICC) lie in the myenteric region, between the circular and longitudinal muscle layers. ICC are connected by gap junctions to surrounding ICC and also with circular and longitudinal smooth muscle cells, forming a large electrical syncytium. Damage of the ICC causes a disorder in the patterns of rhythmic activity. Isolated ICC produce a rhythmic oscillation of the membrane potential. All these observations have led to the suggestion that ICC may be the pacemaker cell responsible for intestinal activity. Gastric smooth muscles generate slow oscillatory membrane potential changes (slow waves) and spike potentials. The activity is considered to be linked to the metabolism in the cell. Three types of cells located in the gastric wall (circular and longitudinal smooth muscle cells and ICC) produce synchronized electrical responses with different shapes. The electrical responses appear to originate in ICC and then spread to the smooth muscle layers, indicating that ICC may also be the pacemaker cells responsible for gastric activity. However, isolated circular smooth muscle tissues spontaneously generate regenerative potentials, suggesting that there are at least two sites for the initiation of spontaneous activity in the stomach. Regenerative potentials persist in the presence of Ca-antagonists and are inhibited by agents which disrupt intracellular Ca(2+) homeostasis. Depolarization of the membrane elicits regenerative potentials after a long delay and the potentials have long refractory periods. This suggests that an unidentified 2nd messenger may be formed during the delay between membrane depolarization and the initiation of a regenerative potential. In gastric muscles of mutant mice which do not express inositol trisphosphate (InsP(3)) receptors, spike potentials but not slow waves are generated, suggesting the possible involvement of InsP(3) in the initiation of spontaneous activity.

Spontaneous electrical rhythmicity in cultured interstitial cells of cajal from the murine small intestine

1. Interstitial cells of Cajal (ICC) are pacemaker cells in the small bowel, and therefore this cell type must express the mechanism responsible for slow wave activity. Isolated ICC were cultured for 1-3 days from the murine small intestine and identified with c-Kit-like immunoreactivity (c-Kit-LI). 2. Electrical recordings were obtained from cultured ICC with the whole-cell patch clamp technique. ICC were rhythmically active, producing regular slow wave depolarizations with waveforms and properties similar to slow waves in intact tissues. 3. Spontaneous activity of c-Kit-LI cells was inhibited by reduced extracellular Na+, gadolinium, and reduced extracellular Ca2+. The activity was not affected by nisoldipine. Voltage clamp studies showed rhythmic inward currents that were probably responsible for the slow wave activity. The current-voltage relationship showed that the spontaneous currents reversed at about +17 mV. These observations are consistent with the involvement of a non-selective cation current in the generation of slow waves, but do not rule out contributions from other conductances or transporters. 4. A Ba2+-sensitive inwardly rectifying K+ current in c-Kit-LI cells that may be involved in slow wave repolarization and maintenance of a negative potential between slow waves was also found. Similar pharmacology was observed in studies of intact murine intestinal muscles. 5. Cultured ICC may be a useful model for studying the properties and pharmacology of some of the ionic conductances involved in spontaneous rhythmicity in the gastrointestinal tract.

Ionic basis for spontaneous depolarizations in isolated smooth muscle cells of canine colon

The type of cell that serves as the pacemaker in the colon is presently unknown. This study evaluated the ionic basis of spontaneous depolarizations in circular smooth muscle cells isolated from canine colon using whole cell voltage and current clamp techniques. Increasing temperature increased the probability of observing spontaneous depolarizations, depolarized the resting membrane potential (RMP), and increased Ca2+ and K+ currents. Spontaneous depolarizations occurred as rhythmic events, in bursts, or as isolated events. Varying the holding potential from -100 to -40 mV inhibited a component of inward current thought to be necessary for spontaneous depolarizations. The Ca2+ channel blockers, nickel and nisoldipine, inhibited spontaneous depolarizations. Nickel caused a hyperpolarization, whereas nisoldipine did not affect RMP. Ouabain depolarized the RMP and inhibited spontaneous depolarizations. The K+ channel blocker, tetraethylammonium, depolarized the RMP and lengthened the duration of spontaneous depolarizations. The key finding is that single colon circular smooth muscle cells are capable of generating spontaneous depolarizations similar to those described for slow waves in intact tissues and that a temperature- and nickel-sensitive inward current is essential for spontaneous activity.

Role of the sodium pump in pacemaker generation in dog colonic smooth muscle

1. The role of the Na+ pump in the generation of slow wave activity in circular muscle of the dog colon was investigated using a partitioned 'Abe-Tomita' type chamber for voltage control. 2. Blockade of the Na+ pump by omission of extracellular K+, by ouabain, or the combination of 0 mM-Na+ and ouabain, depolarized the membrane up to approximately -40 mV and abolished the slow wave activity. Repolarization back to the control membrane potential by hyperpolarizing current restored the slow wave activity. 3. Slow waves continued to be present in 0 Na+, Li+ HEPES solution. 4. The depolarization induced by the procedures to block Na+ pump activity was associated with an increase in input membrane resistance. 5. Voltage-current relationships show the presence of an inward rectification. 6. Reduction of temperature depolarized the membrane, and decreased the slow wave frequency and amplitude. The slow wave amplitude was restored by repolarization of the membrane. 7. Brief depolarizing pulses evoked premature slow waves. Brief hyperpolarizing pulses terminated the slow waves. 8. We conclude that abolition of slow wave activity by Na+ pump blockade is a direct effect of membrane depolarization and that the Na+ pump is not responsible for the generation of the slow wave. 9. Our results are consistent with the hypothesis that pacemaker activity in smooth muscle is a consequence of membrane conductance changes which are metabolically dependent.

Ionic basis of pacemaker generation in dog colonic smooth muscle

1. The ionic basis of the slow waves in the circular muscle of the dog colon, in particular the ionic conductances involved in their initiation, were investigated by measuring intracellular electrical activity in the Abe-Tomita-type chamber for voltage control. 2. The depolarization that initiates the slow wave activity could be evoked by an increase in inward current and/or by a block of outward current. According to previous work, inward current could be carried by Na+, Cl-, and Ca2+ ions; K+ ions would carry outward current. 3. The Na+ channel blocker tetrodotoxin (5 x 10(-7) M) did not affect the slow wave amplitude nor its rate of rise. After omission of Na+, by replacing Na+ with N-methyl-D-glucamine, large slow waves continued to develop although some changes in slow wave characteristics occurred. 4. Replacement of 91% of the Cl- by isethionate decreased the slow wave frequency and increased the slow wave amplitude. However, NaCl substitution by sucrose increased the slow wave frequency and decreased the slow wave amplitude. 5. Slow wave activity continued to develop after blockade of Ca2+ influx by D600 (10(-6) M) or CoCl2 (1-3 mM). D600 and Co2+ did not affect the membrane potential but reduced the slow wave amplitude and abolished the plateau potential. Slow waves were abolished after omission of extracellular Ca2+ (plus 1 mM-EGTA). This suggests that Ca2+ influx is probably not necessary but extracellular presence of Ca2+ ions is indispensible for the slow wave generation. 6. The combination of 0 Na+, Li+ HEPES solution, by replacing Na+ with Li+, plus D600 depolarized the cells (up to approximately -40 mV) and abolished slow wave activity. This effect was voltage dependent since repolarization caused slow waves to return. 7. Abolition of the slow wave activity was also obtained by current-induced depolarization to approximately -40 mV. However, during high-K+-induced depolarization (to approximately -40 mV) high amplitude (16 mV) slow waves were still present, slowing that the voltage dependence of the slow waves was shifted positively. This effect probably occurs due to modification by extracellular K+ of a voltage-dependent K+ conductance, which would suggest that a K+ conductance is involved in slow wave generation. 8. In conclusion, slow waves are generated by cyclic membrane conductance changes, which are dependent on the presence of extracellular Ca2+ ions and on the membrane potential. Our data are consistent with the hypothesis that slow waves are initiated by the blockade of a K+ conductance.

Disturbed hypothalamic control of Na,K-ATPase: a cause of somatic symptoms of depression

Current theories of the causes of somatic symptoms in depression neglect evidence for a general inhibition of Na,K-ATPase. Experimentally, sodium pump inhibition is associated with depolarization of electrically active tissue: the threshold of peripheral nerves is decreased and smooth muscle function altered. Symptoms compatible with these changes are found both in depression and intoxication with digoxin, which inhibits Na,K-ATPase. The hypothalamus contains inhibitors of Na,K-ATPase which are capable of producing depolarization. In depression, changes in hypothalamic activity may increase endogenous inhibition and contribute to the somatic symptoms.

Mechanism of action of angiotensin II on excitation-contraction coupling in the rat portal vein

1 The action of angiotensin II (At II) has been studied on the electrical and mechanical activity of the vascular smooth muscle of the rat portal vein.2 At low concentrations (between 5 x 10(-10) and 10(-9) M) At II induces an acceleration of spontaneous action potential (AP) discharge without change in the resting membrane potential. The frequency and size of the associated contractions are simultaneously augmented. Under these conditions the size of the spikes is not affected, thus suggesting that At II triggers the release of Ca(2+) from internal stores.3 The increase in AP discharge rate produced by low concentrations of At II results from an acceleration of the pacemaker potential. Furthermore, in the presence of 10 mM tetraethylammonium (TEA), there is an acceleration of the repolarizing phase of AP.4 Ouabain (10(-3) M) inhibits the increase in rhythmic activity induced by low concentrations of At II (in the presence of 10 mM TEA), thus suggesting that the Na-K pump is directly or indirectly involved in this action of the peptide.5 At higher concentrations, At II produces a concentration-dependent depolarization with an EC(50) of 1.2 x 10(-8) M and a maximum of 10(-7) M. The associated contraction has an EC(50) of 3.3 x 10(-8) M and a maximum of 3 x 10(-7) M.6 Ouabain (3 x 10(-3) M) depolarizes the cell membrane. Under these conditions, At II (10(-7) M) has a slight depolarizing effect, but it still produces a large tonic contraction.7 It is concluded, that At II acts on different steps of excitation-contraction coupling, depending on the concentration. At low levels, the peptide mainly accelerates spike discharge, through a mechanism involving the Na-K pump. At higher concentrations, At II depolarizes the cell membrane. The contraction is then activated by the influx of Ca(2+) due to secondary AP discharge and the release of Ca(2+) from intracellular stores. Pharmacomechanical coupling has an important role in the triggering of contractions both at high and at low concentrations of At II.

Prolonged exposure to ouabain eliminates the greater norepinephrine-dependent calcium sensitivity of resistance vessels in spontaneously hypertensive rats

The effects of 30 minutes of exposure to ouabain on calcium sensitivity have been investigated in two types of resistance vessels from 12 pairs of spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats. Branches of the superior mesenteric and femoral arteries, with internal diameters of about 200 micrometer, were mounted as ring preparations in a myograph capable of measuring their isometric wall tension. Dose-response curves for calcium upon norepinephrine stimulation were determined under conditions where neuronal uptake was eliminated. Initially, when stimulated with norepinephrine, the SHR vessels from both locations were more sensitive to calcium and had stronger contractions than their controls. The addition of ouabain (1 mM) to the relaxed vessels immediately elicited a moderate, transient contraction in the branches of the femoral artery, whereas no response was observed in the mesenteric vessels. Although the addition of ouabain to activated vessels produced an immediate potentiation of the response, prolonged (30-minute) exposure to ouabain reduced active tension development upon norepinephrine stimulation in all vessels. The reduction was greatest in the SHR vessels, so that, under these conditions, the norepinephrine-activated calcium sensitivity of corresponding SHR and WKY vessels was similar. By contrast, responses to norepinephrine in high potassium solution were unaffected. The results suggest that under normal conditions, SHR vessels may have a specific increase in the permeability of the norepinephrine-activated calcium channels. Prolonged exposure to ouabain appears to reduce the permeability of these channels, providing an explanation for why this treatment eliminates the difference in calcium sensitivity of the SHR and WKY vessels.

The electrical activity recorded from smooth muscle of the circular layer of the human stomach

The membrane properties of circular muscles of 55 human stomachs were investigated by microelectrode and double sucrose gap methods. The membrane potential of the circular muscle of the corpus region was -57 mV and no regional difference was evident as compared with tissues from the antrum and cardia. The stomach muscle presented cable like properties, and the length constant measured in the corpus region was 1.34 mm. The circular muscle of all regions of the stomach exhibited slow waves. The amplitude and duration of slow waves varied markedly (the mean values were 18 mV and 6 s, respectively). The Q10 value for the slow wave was 2.4. The slow wave could be divided into two different components (first and second component) by application of electrical current or by using solutions with various ionic environments. Na ions had more effect on the spike component and Ca ions on the second component. The generation of the first component of the slow wave was blocked by either Na-free, K-free, Ca-free, or Cl-deficient solution but this component reappeared by application of outward current pulse, except in Cl-deficient solution. These results suggest that the generation of slow wave depends on more than one type of ion and that metabolic factors do indeed play a role. Membrane properties of the human stomach were compared with those of the guinea-pig stomach.

Electrophysiological properties of human oviduct smooth muscle cells in dissociated cell culture

Intracellular recordings were made from human oviduct smooth muscle maintained in cell culture. Solitary cells isolated from one another and cells in contact with one another retained electrical properties of smooth muscle in vivo. Membrane potential of solitary cells and connected cells was -35 mV. Connected cells formed electrotonic junctions which transmitted current from one cell to another. This current spread was responsible for differences in input resistance and time constant in solitary cells, 66 Momega and 96 msec, compared to connected cells, 26 Momega and 56 msec. All cells expressed delayed rectification to depolarizing current pulses. Some cells generated action potentials spontaneously or in response to intracellular current pulses. Action potentials were abolished by cobalt or by EGTA. Slow wave potentials, 5 . 20 mV in amplitude, occurred continuously once every 15 to 45 seconds in connected cells.

Rhythmic contractions induced by vagotomy in the fundus of rat stomach

Continuous rhythmic contractions were observed in longitudinal muscle strips of fundus obtained from the chronically vagotomized rat stomach. These vagotomy-induced rhythmic contractions (VRC) were not blocked either by tetrodotoxin (3 x 10(-6) M) or atropine (3 x 10(-7) M), indicating that the VRC was myogenic in origin. It was also found that the VRC had the following characteristics. 1) The VRC blocked either by 10(-5) M D600 or by Ca2+ deprivation from the medium. 2) On replacement of Na+ with sucrose or of K+ with Na+, the tone of fundus strips was concomitantly elevated with the cessation of the VRC. 3) 10(-5) M ouabain blocked the VRC. 4) The VRC was blocked by cooling. 5) The VRC disappeared under anoxic conditions. Based on these results possible mechanisms of the VRC are discussed in relation to an energy dependent activity of the cell membrane.

Interaction between longitudinal and circular muscle in intestine of cat

1. Slow waves recorded from isolated longitudinal muscle averaged 13 mV and had slow rate of rise (0.04 V/sec) whereas when recorded from intact segments the amplitude averaged 27 mV and the rate of rise was more rapid (0.09 V/sec), often with a notch between the initial peak and the plateau. Membrane potentials of longitudinal muscle were similar in isolated and intact preparations (- 66 mV). Resting potentials of circular muscle averaged - 67 mV.2. Small bundles of circular muscle tested in the double sucrose gap produced activity, either spontaneously or in response to stimulation, which fell into three categories: fast spikes (50-200 msec duration), slow spikes (1-5 sec duration), and small graded responses. The duration of fast spikes could be increased severalfold by the addition of TEA; the graded responses were converted to full-sized spikes by TEA.3. Treatment of circular muscle with Ca-free Krebs solution eliminated spikes, and in intact preparations reduced the amplitude and rate of rise of slow waves and eliminated the notch on slow waves.4. Current-voltage curves of longitudinal muscle show delayed rectification in the depolarizing quadrant; similar curves of circular muscle show anomalous rectification, i.e. a region where a very small current causes a large voltage change.5. Non-polarized electrotonic coupling between longitudinal and circular layers indicates low-resistance pathways. Apparent space constants of longitudinal muscle are greater when attached to circular muscle than when isolated.6. It is concluded that small slow potentials originate rhythmically in longitudinal muscle, that these spread passively to circular muscle where a regenerative amplification occurs which depends on Ca conductance and the amplified slow waves spread back to the longitudinal layer. In the intact intestine pacemaking is, therefore, separate from propagation and the circular muscle provides the bulk of depolarizing current for propagation.

Prolonged potentials in gastrointestinal muscles induced by calcium chelation

When stomach muscles of skate, toad, or frog or intestinal muscle of cat are treated with Ca-free physiological solutions containing 2-5 mM EGTA or EDTA, spontaneous spikes and slow waves disappear reversibly. With continued treatment, depolarization of 25-30 mV from resting potentials of -65 mV occurs and rhythmic prolonged potentials of several seconds duration appear. They show rapid depolarization to near zero and rapid repolarization and they may continue for several hours. The prolonged potentials disappear when Na is replaced by Li, Tris, or choline. They are insensitive to TTX. The EGTA-induced waves are abolished by Mn, Co, La, verapamil, and D 600. After 10-15 min in 5 mM EGTA, voltage-current and abolition of anomalous rectification. It is concluded that when bound Ca is removed by a chelator, nonspecific reduction in resistance occurs and Na ions may enter rhythmically through channels normally used by Ca.

Effects of sodium, potassium and calcium ions on the slow wave in the circular muscle of the guinea-pig stomach

1. The contribution of Na, K and Ca ions to the generation of slow waves in the circular muscle of the guinea-pig stomach was studied.2. The slow waves had a lower, potential-independent (first) and an upper, potential-dependent (second) component. Reduction of the external Na prolonged the first component, but complete removal of Na depolarized the membrane and caused deterioration of the slow wave.3. Readmission of Na (5-10 mM) restored the slow wave; this action was not abolished by ouabain.4. Removal of K depolarized the membrane and slightly reduced the amplitude and duration of the slow waves. Readmission of K hyperpolarized the membrane and increased the amplitude and duration of the slow waves, particularly of the first component. Ouabain blocked the effects on the membrane potential, but not the effects on the slow waves.5. An increase in extracellular Ca prolonged the first component and reduced the frequency. Removal of extracellular Ca abolished the slow wave activity. Excess Ca enhanced, and low Ca reduced the effects of altering the concentrations of external K.6. It is concluded that the ouabain-sensitive Na-K pump may not be directly involved in generating slow waves, but that some other metabolic process is involved, which is regulated to a large extent by Ca, and possibly also by Na and K.

Na+-K+-ATPase inhibition stimulates PGE release in guinea pig taenia coli

Inhibition of the plasma membrane enzyme Na+-K+-ATPase by ouabain zero extracellular K+, or low extracellular Na+, markedly augmented prostaglandin E release from the guinea pig taenia coli. Data suggest this phenomenon may be linked directly to Na+-K+-ATPase or Na+ pump activities, or changes in intracellular K+ concentration. The augmented prostaglandin E release was not due to changes in intracellular Na+, Ca2+, pH, or membrane potential, resulting from Na+ pump inhibition. The characteristics of the plasma membrane may exert a control on prostaglandin E release in this smooth muscle.

A study of pace-maker activity in intestinal smooth muscle

1. Electrical activity of longitudinal muscle from cat intestine was recorded in the double sucrose gap.2. Approximately 20% of the preparations demonstrated slow, spontaneous fluctuations of membrane voltage, slow waves. This activity, although quite uniform in a given preparation, showed considerable inter-preparation variation with respect to amplitude, frequency and wave form.3. Application of steady hyperpolarizing current decreased slow-wave frequency and increased slow-wave amplitude while depolarizing currents increased frequency and decreased amplitude.4. Some preparations with no spontaneous slow-wave activity developed slow waves when the membrane was hyperpolarized into a given range which, depending on the preparation, varied in size from 10 to 40 mV. Step or ramp depolarization of the membrane from hyperpolarized levels triggered slow waves in some preparations.5. When the membrane potential of a slow-wave generating preparation was clamped at the resting potential, spontaneous inward-directed current transients were observed.6. No changes in membrane conductance were observed during the course of a slow wave.7. The slow-wave pattern was simulated for individual preparations by applying the membrane current measured under voltage clamp to the passive membrane resistance and capacitance measured independently under current clamp.8. In addition to the defined slow-wave activity, voltage-dependent oscillations in membrane potential were sometimes observed.9. Application of 10(-5)M ouabain irreversibly blocked slow waves and produced a membrane depolarization equal to or slightly greater than the slow wave crest. Repolarization of the membrane to the resting potential, or hyperpolarization, failed to restore slow-wave activity.10. Removal of external potassium produced a reversible sequence of events almost identical to those following ouabain application.11. Replacement of 50% of the external sodium chloride with sucrose produced no changes in slow-wave activity with respect to rates of rise or fall, maximum amplitude or frequency. Sucrose replacement of all external sodium chloride eliminated slow waves after 5 min; however, activity could be restored by a slight hyperpolarization. Longer exposures to the modified bath abolished activity.12. Following a conditioning exposure to potassium-free Krebs solution, readmission of potassium at normal concentration produced a mean hyperpolarization of 20.5 mV and in spontaneous preparations an arrest of activity.13. Pump current in sodium-loaded, non-spontaneously active preparations was measured by voltage clamp and was observed to be voltage-dependent.14. The results of this study indicate that an electrogenic pump is present in longitudinal muscle of cat duodenum, and that oscillations in the level of pump current produce slow waves.

Effects of tetraethylammonium chloride on the membrane activity of guinea-pig stomach smooth muscle

1. The effects of tetraethylammonium (TEA) on the membrane activity of the antral circular muscle of the guinea-pig stomach were investigated with micro-electrode and double sucrose gap methods.2. In a concentration of 1-1.5 x 10(-3) g/ml. (3-5 mM), the membrane potential was not influenced; the membrane resistance measured by inward current pulses remained the same but the rectifying property of the membrane was suppressed.3. TEA (1-1.5 x 10(-3) g/ml.) enhanced the spike amplitude markedly even from fibres which generated graded responses.4. TEA (1-1.5 x 10(-3) g/ml.) did not increase the maximum rate of rise of the spike but decreased the maximum rate of fall of the spike markedly.5. In Na-free (Tris or sucrose) solution, in K-deficient and excess-K solutions, TEA (1-1.5 x 10(-3) g/ml.) suppressed the rectifying property of the membrane and enhanced the spike amplitude.6. Atropine (10(-6) g/ml.) had no effect on the enhancement of the spike amplitude produced by TEA.7. The minimum concentration of Ca ions required for the effect of TEA on the spike amplitude was one fifth of the normal concentration. TEA also enhanced the spike amplitude in Sr-Krebs.8. The possible role of TEA on the membrane activity is considered to be due to suppression of the K conductance when the membrane is depolarized. Alternative possible roles of TEA on the spike amplitude are also discussed.